Volcanically hosted venting with indications of ultramafic influence at Aurora hydrothermal field on Gakkel Ridge

The Aurora hydrothermal system, Arctic Ocean, hosts active submarine venting within an extensive field of relict mineral deposits. Here we show the site is associated with a neovolcanic mound located within the Gakkel Ridge rift-valley floor, but deep-tow camera and sidescan surveys reveal the site to be ≥100 m across—unusually large for a volcanically hosted vent on a slow-spreading ridge and more comparable to tectonically hosted systems that require large time-integrated heat-fluxes to form. The hydrothermal plume emanating from Aurora exhibits much higher dissolved CH4/Mn values than typical basalt-hosted hydrothermal systems and, instead, closely resembles those of high-temperature ultramafic-influenced vents at slow-spreading ridges. We hypothesize that deep-penetrating fluid circulation may have sustained the prolonged venting evident at the Aurora hydrothermal field with a hydrothermal convection cell that can access ultramafic lithologies underlying anomalously thin ocean crust at this ultraslow spreading ridge setting. Our findings have implications for ultra-slow ridge cooling, global marine mineral distributions, and the diversity of geologic settings that can host abiotic organic synthesis - pertinent to the search for life beyond Earth.


Sincerely,
Reviewer #2 (Remarks to the Author): Dear authors, the study investigates the geological setting of the high-temperature venting Aurora hydrothermal field on the ultra-slow spreading Gakkel Ridge, which is hardly accessible due to the Arctic ice cover. The study presents near-bottom sidescan sonar data, visual seafloor observations and geochemical plume analyses with the aim to identify the host rock lithology in the subseafloor of the Aurora vent field.
The sidescan data is interpreted for reflectivity patterns, which are linked to seafloor photographs and other ground-truthing observations. Both show the Aurora field to be hosted in highly-reflective, relatively fresh basalt that piles up as a ridge-like volcanic feature that is otherwise surrounded by lowreflectivity, heavily sedimented seafloor. Visual observations reveal a relatively large spatial extent of the field and a variety of venting stages from high-temperature fluids to extinct vent sites. This, together with the ultra-slow spreading nature of the hosting mid-ocean ridge, is used to hypothesise a stable, long-lasting hydrothermal circulation cell underlying the Aurora hydrothermal field. Such stable circulation is typically known from ultramafic sites and contradicts the fresh, basaltic surface lithology observed in the remote sensing data. Therefore, the authors hypothesise an ultramafic lithology in the subsurface of the hydrothermal field underneath a basaltic cover.
An ultramafic host rock signature is identified in the fluid composition of the non-buoyant hydrothermal plume above the hydrothermal field. Analysed He-isotope, dissolved Methane and dissolved Mn concentrations in the plume are assumed to reflect those at the seafloor vent sites. The geochemical ratio of Methane vs. Mn at Aurora compares to ratios known from other ultramafic sites at other ultraslow and slow-spreading mid-ocean ridges.
The study concludes that the Aurora hydrothermal deposit is basalt-hosted at the surface, but the underlying, subsurface circulation cell is dominated by ultramafic host rocks that contribute substantial Methane to the hydrothermal fluid.
Considering the Arctic ice-cover above the Gakkel Ridge, the presented seafloor observations and remote sensing data of the Aurora hydrothermal field are spectacular. The article is well written and the argumentation is good to follow.
The scientific relevance of the characterized geological setting could be better highlighted (see comments below).
I recommend the paper to be published after minor revisions. I split my feedback into some general questions for each section and some line by line comments.
My expertise lies in remote sensing of the seafloor around hydrothermal fields, so my review focusses on the analyses of the sidescan data, seafloor observations, and bathymetric maps.
Best regards, Meike Klischies ---General questions and comments: Introduction: The introduction needs a sentence explaining, why it is important to better characterize the geological setting of the Aurora hydrothermal field. What gap does the presented findings close and how do they affect our general view on the geological settings of hydrothermal systems? Also, the study demonstrates clearly that remote sensing alone can only characterize the surficial geological settings, but geochemical analyses of fluids etc. are needed to understand the entire hydrothermal circulation cell including the (deep) subsurface. This should be introduced here.
The history of exploring the Gakkel Ridge (first paragraph) could be shortened.
Results: The extent of the field and the reflectivity patterns in the sidescan image are described in much detail in the text and figure captions. To better illustrate these descriptions, I would wish a (geological) map of the area showing the field's extent and the different acoustic and/or geological entities. Discussion: To better highlight the significance of the findings, add a sentence about the need for geological characterizations of hydrothermal fields; for example, how do the findings affect the exploration for hydrothermal deposits at other (ultra-slow spreading) mid-ocean ridges.
Figures: The maps in the figures, including the sidescan image, should get a consistent layout (same colour ramp for the bathymetry, coordinate frame, scale bar, north arrow or coordinate grid lines).

Line by line comments:
Line 66: Refer here to the bathymetry in Figure 1.
Line 77: This reads a bit confusing, if you do not know what OFOBS is or is equipped with. Maybe, give a short list of the main instruments of the platform, which would prepare for the following analyses of the sidescan data and photo footage.
Line 80: Is it necessary to mention the failed deployments (OFOBS5) here? Or do you list it, because it provided you with ground truth at distance to the Aurora field? If the latter is correct, rephrase the sentence and state how you used these deployment's data.
Line 91: Could the coring stations be plotted as ground truthing in the Fig.1 or in the sidescan map in Fig.2a?
Line 192: Lucky Strike's Methane:Mn ratio is described here as an exception. Can the ratio of the Aurora plume also be an exception? In the bathymetry, the setting of the Aurora field within the (partly rifted?) Aurora neovolcanic mound appears similar to the setting at Lucky Strike, e.g. Escartín et al. (2014).   Fig.1 to show the regional setting of the Aurora field and neovolcanic mound.   The work of German et al in the NCOMMS-22-12440-T ms provides a study of the Aurora hydrothermal vent field along Gakkel Ridge using a series of OFOBS (combined towed HD camera and high-frequency sidescan sonar) surveys and CTD/rosette samples. The authors show images of an active black smoker site at the western most tip of the Aurora Seamount with hydrothermal plume CH4/dMn (dMn stands for dissolved Manganese) ratios that follows the same trend than the Rainbow hydrothermal plume. They also show evidences for a highly sedimented rift valley, with a sulfide deposit district that may extend laterally over a hundred meters scale. Although the work of German et al does bring new data and information about hydrothermal activity along a ultra-slow spreading ridge, I find that the ms needs improvement before it can be published in Nature Communications. I detail hereafter my main concerns.

Major comments:
Based on the anomaly detected by biogeochemical sensors (temperature and H2S anomalies), the authors performed OFOBS transects where the seafloor images showed active chimneys with black smoker vents and basaltic pillow lavas. The authors performed towed CTD/rosette casts across the active vent site and argue that the CH4/dMn ratio observed in the plume falls into the range of ultramafic hydrothermal plumes previously observed (Rainbow, Logatchev and Ashadze). They use this CH4/dMn ratio close to unity to suggest that the Aurora vent site is under ultramafic influence due to a thin crust cover in the ultra-slow spreading ridge context of the Gakkel ridge.
My main concern with the ms lies in the lack of argument backing up the ultramafic influence hypothesis. Over the 4 examples of ultramafic vent sites known (Rainbow, Logatchev, Ashadze, Kairei) and used by the authors to define the ultramafic range, they discard one of them (the Kairei vent field) without explanation (l.206 -209). Aurora lies on a neovolcanic basalt mound, and images from the OFOBS show basalts pillows, and smooth thickly-sedimented rift-valley. No dredge samples, no seafloor images nor geophysical data demonstrate that the upper mantle outcrops or is very near the surface. And given the Aurora seamount bathymetry, there is at least 400m (Aurora seamount height) of basalt constituting the upper layer of the crust. Some authors (Cochran et al.2003 https://doi.org/10.1029/2002JB001830) show that the crust thickness can be highly variable at Gakkel ridge (e.g. up to 1km east of 32°E to up to 3km west of 32°E to at least 8°E). Further west (i.e. western volcanic zone, 7°W-3°E) -where the Aurora vent site lies -the segment reflects abundant volcanism (Michael et al., 2003 https://doi.org/10.1038/nature01704) with a spreading rate around 13.5-14.5mm.yr-1. Additionally, peridotites were not dredged from the Western Volcanic Zone of the Gakkel Ridge (WVZ -Patterson et al., 2021 https://doi.org/10.1016/j.lithos.2021.106107). Is there any geological clues that would support a thinner crust (i.e. <1km) at the Aurora mound that would support German et al. interpretation? . Given that hydrothermal convection cells are around 1 to 2 km in diameter, influence of mantle rocks in hydrothermal fluid circulation due to a thin ocean crust cover still appears a bit hypothetical. Were there any other evidences that could back up the authors' hypothesis: significant hydrogen concentrations in the water column for instance?
Minor concern: The authors focused their OFOBS survey around the black smoker site, but from the biogeochemical sensors, other lower, but still significant, temperature and H2S anomalies were observed during the 2014 cruise (OFOS PS86/065, p66 of the cruise report). How can the authors be sure that there is only a single active high temperature site? None of the 2019 OFOBS surveys were close enough to the aforementioned anomalies to provide images of the seafloor.

Responses to Reviewer Comments
A recurrent theme evident from reading all three reviews is that we tried to fit too much "assumed knowledge" into our original manuscript (~2500 words for [redacted]) and we have now extended the text to ensure that we have allowed enough space to address the reviewers' key issues. The comments of Rev.2 and especially Rev.3 provided perceptive insights that helped us focus on how best to take advantage of the extra space available in Nature Communications -both to expand on critical ideas and to add new information, including a new section at the end of the discussion on the broader significance of our work. Below, we present a detailed discussion of the changes that we have made, together with additional supporting comments, in response to each point raised.
Particular changes that we have made to the Discussion, responsive to specific concerns highlighted in the editor's cover letter, include: a) Accentuating the significance of the Aurora site's size, hence, requisite time-integrated heat flux for formation as an important (but perhaps overlooked) component of our original arguments (Discussion,. b) Explaining the basis for excluding 2 "outlier" data-points (both Lucky Strike and Kairei are considered anomalous at the global scale, irrespective of this study) c) Introducing two new, independent lines of argument to support our hypotheses, one microbiological (l.293-309) and one petrological (l.310-329 & Fig.5) d) Adding a new section on the broader significance of our work (also in the abstract) that highlights the potential relevance of our findings; these range from new marine mining opportunities (l.340-360) to helping guide the search for life beyond Earth (l.361-382).

First of all, the involvement of ultramafic rocks in a neovolcanic hydrothermal system on an ultraslow-spreading mid-ocean ridge seems hardly surprising…
We agree that ultramafic hosted systems should, in general, be expected along slower spreading ridges. Nonetheless, what we still consider surprising is that all prior work at Aurora (including two prominent, well-cited Nature articles) had predicted the exact opposite for this site. (See also Rev.3's surprise that our hypotheses appear to be in stark contradiction to "received wisdom" based on pre-existing literature from the same study area). Importantly, our work implies a (globally) novel setting for seafloor venting with wide-reaching implications. We elaborate on this (as encouraged by Rev.2, point 2.6) in the revised Discussion at l.331-382.

Their conclusion is solely based on 1) the presence of high-temperature blacksmoker-type venting at Aurora and 2) the presence of a certain CH4-Mn-ratio that they claim cannot be explained by sediment interaction.
This reviewer appears to have overlooked a key additional point that informed the hypotheses that we presented -the unusual size and, hence, the large time-integrated heat-flux required to generate the vent field observed at Aurora. Admittedly this was not discussed in much detail within the much shorter original manuscript. While the composition (CH4/Mn ratio) of the plume argues against simple water-rock reactions with basaltic rocks at Aurora, the size of the field provides a separate and independent argument for why Aurora can no longer be considered a 'typical' short-lived neovolcanic-hosted system. We have now sought to make this point more explicit, including revisions to the abstract (l.29-30, l.33-35) as well as a thorough re-write of the opening section of the Discussion (l. . We have also expanded our title (Nature Communications has longer word-limits than [redacted]) to emphasize that what we present here are testable hypotheses that could explain our novel and surprising data. Further fieldprograms would be required for firmer conclusions to be reached on the same topic.

The argumentation with respect to excluding sedimentary sources for the CH4-Mnratio is, however, weak... Do [other sedimented] vent sites show a similar CH4-Mn-ratio?
In answer to Rev.1's question, MOR vents influenced by any kinds of sediment typically exhibit CH4/Mn ratios that are 1-2 orders of magnitude higher than the values reported here (l.271-272). But we could have done a much better job in the original manuscript explaining that the only reason why we mentioned sediments at all was to make a clear distinction between our results and those from Aurora's nearest known neighbor, Loki's Castle, on the Mohns Ridge.
Recognizing that Aurora is also set on an ultra-slow spreading ridge, in the ice-covered Arctic, it is clear that neither back-arc basins (e.g. our own original lines 213-216) nor organic-rich sedimented systems from the Gulf of California (introduced by Rev.1) present viable analogs for Aurora. We do not need to consider CH4/Mn values to reach that conclusion. In the revised manuscript, therefore, we have chosen to focus upon drawing out a more explicit comparison with Loki's Castle (l.270-292) a site that does exhibit clear evidence of terrigenous-sedimentinfluence. We acknowledge that the Loki's Castle values overlap with values for mid ocean ridge vents overlain by terrigenous sediments but that, because all of those values fall FAR outside the range presented here (Fig.4), we can be confident that sediment influence has no role to play at Aurora.

Additional data providing further evidence for the ultramafic involvement is needed.
We address this more completely in response to Rev.3 (points 3.2, 3.3). In brief: in addition to (i) the time-integrated heat-flux required to generate the size of the Aurora field and (ii) the characteristic CH4/Mn ratio of its overlying plume, we now present (iii) evidence from basalt compositions, which are distinct from most of the Gakkel Ridge's Western Volcanic Zone, that are indicative of anomalously thin basaltic crust beneath Aurora (3.2) and (iv) results from a new companion study (now in revision for Nature Microbiology) that suggests H2 release from the Aurora hydrothermal field, consistent with subsurface ultramafic influence (3.3).

Considering the Arctic ice-cover above the Gakkel Ridge, the presented seafloor observations and remote sensing data of the Aurora hydrothermal field are spectacular.
We appreciate that this reviewer recognized how unique and exciting these data sets are.

Introduction: The introduction needs a sentence explaining, why it is important to better characterize the geological setting of the Aurora hydrothermal field. What gap does the presented findings close and how do they affect our general view on the geological settings of hydrothermal systems?
Done -see para 2 of revised Introduction (l.59-71)

Also, the study demonstrates clearly that remote sensing alone can only characterize the surficial geological settings, but geochemical analyses of fluids etc. are needed to understand the entire hydrothermal circulation cell including the (deep) subsurface. This should be introduced here.
Agreed -but we have deferred any explicit call out of the benefits of our multi-disciplinary approach to the Discussion (e.g. opening of second section, l.202-203) where we can highlight concrete examples of the value gained by bringing multiple threads (seafloor surveys, water column chemistry, microbiology, petrology) together to provide novel insights beyond what any one approach could reveal in isolation.

The history of exploring the Gakkel Ridge (first paragraph) could be shortened.
Done

Results: I would wish a (geological) map of the area showing the field's extent and the different acoustic and/or geological entities.
Done -see new Fig.2b (geologic interpretation) to match Fig.2a (sidescan survey data)

Discussion: To better highlight the significance of the findings…
Done -see entire new section added to the Discussion to address this, l.331-382

The maps in the figures, including the sidescan image, should get a consistent layout
All map figures have been revised along the lines suggested (see detailed comments below)

Rev 2. Line by line comments:
Line 66: Refer here to the bathymetry in Figure 1. Done -l.52

Line 77: Maybe, give a short list of the main instruments of the platform, which would prepare for the following analyses of the sidescan data and photo footage.
Done -l.83-84

Line 80: Is it necessary to mention the failed deployments (OFOBS5) here?
Good point. OFOBS 05 has been removed, as suggested. Fig.1 or Fig.2? All coring (2014 expedition) was done before the vent-site was located and lie outside of our sidescan survey area. We have updated the text (l.93-99) to clarify this point and explain, instead, that it is our co-registered video-observations that provide the ground-truthing for the sidescan interpretation.

Line 192: Lucky Strike's Methane:Mn ratio is described here as an exception. Can the ratio of the Aurora plume also be an exception?
Aurora *is* an exception to what has been reported previously from all other vents. But we do not consider Lucky Strike to be a viable analog because Lucky Strike site is morphologically anomalous as well as geochemically anomalous compared to all other vents. Morphologically, venting at Lucky Strike is hosted atop a 500-1000m tall axial seamount -no comparable feature occurs anywhere else along the ~2500km of survey that first mapped this section of the Mid-Atlantic Ridge (Detrick et al., J.Geophys.Res., 1995) and none are observed on Gakkel Ridge, either (Michael et al., Nature, 2003). Thus, Lucky Strike differs geologically from all the many tens of basalt-hosted vents worldwide that emit fluids with CH4:Mn ratios < 0.50. The preferred explanation for this is that focused melt extraction and volcanism beneath Lucky Strike must lead to an extremely deep-rooted hydrothermal cell beneath the anomalously thick ocean crustan analogy that does not apply at Aurora. We have amended lines 50-53 of the introduction to make explicit that Aurora lies deep within the Gakkel Ridge rift valley (unlike Lucky Strike and precluding arguments for similarities related to anomalously high magmatic focusing / volcanic effusion rates) and expanded the relevant paragraph of the Discussion section (l.214-230) to present the same rationale outlined in this response.

Line 206: According to Okino et al. (2015), the Kairei vent field is hosted in basalt at the surface. The fluid chemistry at Kairei is explained with a detachment fault at depth and serpentinization of troctolites (Okino et al., 2015, Nakamura et al., 2009)
We are grateful to the reviewer for bringing the Okino et al. (2015) paper to our attention. We have included a more complete consideration of the Kairei site in the revised manuscript (l.246-259) as detailed more fully in our reponse to Rev.3, point 3.1.

Line 225: The argument for an ultramafic subsurface could be strengthened by referring to remote sensing studies of the ultra-slow Southwest Indian Ridge (Sauter et al., 2013).
Done -see revised paragraph opening at l.310-313 and further response to Rev.3, point 3.6. Fig.2a in the figure caption. Fig.2 has been updated to include a geologic map with key ( Fig.2b)

Figure 1: The legend for the inset's colour scheme is missing…
Color scale added to inset as well as labels to show intersection of Lena Trough/Gakkel Ridge Figure 2: The triangles are too large… (Fig.2a). This would be solved by presenting the data (maybe including the survey tracks of the other OFOBS tows) and the interpretation separately in two maps. The seafloor photographs (Fig.2c-d)

Figure 3: I do not understand the benefit for presenting different bathymetry here.
In our original Fig.3a, we presented the bathymetry used to guide the CTD surveys from the same 2014 expedition. We have now replaced that using the same 2019 data as Fig.1.

Figure 4: Label the dashed lines with Lucky Strike and Kairei.
Dashed lines have been labelled.

Logatchev, Ashadze, Kairei) and used by the authors to define the ultramafic range, they discard one of them (the Kairei vent field) without explanation (l.206 -209).
This is an important point (see also Rev.2) and we have expanded this section of the discussion to explain our rationale. Note that there were more than four examples being considered in our original submission. As now described (l.201-292) the global data-set includes *nine* known examples of ultramafic-influenced vent-sites along mid-ocean ridges. Five of those nine sites are high temperature "black smoker" systems located on slow-spreading ridges and, hence, provide the closest plausible geologic analogs to the Aurora site reported here. In the expanded revised version of this section of the Discussion we have added a consideration of the Lost City, Old City and Von Damm hydrothermal fields which we can exclude as candidate analogs for Aurora because (a) they do not host high temperature "black smoker" venting of the type reported here and (b) their CH4:Mn ratios are off the charts compared to Aurora (CH4:Mn >> 10:1) -see l.239-246. In the revised text we have also included a more detailed discussion of why we also exclude the Kairei site from our considerations (l.247-259). Just as is the case for Lucky Strike (Rev.2) -Kairei was already recognized as being both geologically anomalous and geochemically anomalous compared to all other known ultramafic ridge-crest vent-systems, prior to our work at Aurora (Okino et al., 2015). Kairei is the only known example of an ultramafic-hosted system on a medium-fast ridge and the only known seafloor hydrothermal system that has low CH4:Mn ratios in its vent-fluids despite being enriched in both dissolved H2 and dissolved CH4. But the focus of our study is to understand the Aurora hydrothermal system on the Gakkel Ridge rather than to attempt to shed new light on why Kairei is anomalous.
Having established that Kairei cannot provide a relevant analog setting for Aurora, we do not discuss its own unusual characteristics further, here.

Aurora lies on a neovolcanic basalt mound, and images from the OFOBS show basalts pillows, and smooth thickly-sedimented rift-valley… Is there any geological clues that would support a thinner crust?
This is another excellent point that we have been happy to address In the revised manuscript. Petrologically, the basalt data-sets from Michael et al. (2003) already provided evidence (from anomalously low CaO/Al2O3 ratios) for anomalously low levels of melt generation / basaltic crust thickness at the very westernmost limits of the Gakkel Ridge, where it intersects the Lena Trough. We have taken advantage of both the added word count and number of display items allowable for Nature Communications, compared to [redacted], to introduce new Fig.5 to illustrate that point including some confirmatory new analyses of our own. We have revised the Discussion text (l.313-329) to clarify that the ridge-segment hosting the Aurora field, west of 6°W, is petrologically distinct from the remainder of the Western Volcanic Zone (3°E to 5°W).

Were there any other evidences that could back up the authors' hypothesis: significant hydrogen concentrations in the water column for instance?
Informed by the same pre-existing literature as the rest of our community, we did not anticipate any ultramafic influence at Aurora and, hence, did not anticipate a need to measure for dissolved hydrogen while we were on station in 2014. However, microbial samples were collected and in a complementary study, currently being revised for Nature Microbiology, our coauthor Antje Boetius' team have isolated a novel member of the genus Sulfurimonas, which dominates microbial communities within the Aurora plume samples and exhibits traits indicative of an obligate adaptation to hydrogen rich hydrothermal plumes. Further, the report states that bacterial hydrogenases were highly expressed within the plume, but not above or below it, consistent with H2 release from the source hydrothermal field. We have added a discussion of these new data and their implications for our work to the revised Discussion at l.293-309.

How can the authors be sure that there is only a single active high temperature site?
We cannot completely exclude the presence of other hydrothermal sources but as noted in l.158-163 the coherence of the plume data (see also Figs.3&4) all argue for a single coherent vent-source emanating from Aurora. We have reinforced this point in l.77-81 by noting that numerous 2014 OFOS tows over the summit of Aurora failed to locate any other active ventsources. Interestingly, however, the presence of additional active vents would further strengthen our arguments for an unusually large (time-integrated) heat-flux from venting at Aurora, with all sources emitting the same anomalously high CH4/Mn (± high H2) vent-fluids.

Although I do agree that dMn won't be that reactive in early hydrothermal plumes, De Angelis et al, 1993 and Cowen et al, 2002 did show high methane oxidation rates in hydrothermal plume and quick methane removal (1 to 2 km away from the vent site) that severely impacts the CH4/dMn ratios
Actually, examination of Cowen et al (2002) -notably their Fig.5, panel C -only strengthens our argument. Their two stations close to the ridge axis (Stations 6, 9) define a common trend in CH4:Mn without fractionation -just as we present here for Aurora ( ≤3 km range). It is only in their off-axis station (Stn.2) at >5km from the source, that the linear relationship breaks down. Similarly, De Angelis et al (1993) only show attenuation in their dissolved CH4 concentrations at 20km from the source -much greater than the length scales considered here. In our revised text (l.260-269) we acknowledge that biogeochemical transformations in plumes does cause fractionation down-stream -but that for the length scales considered here, so close to the source of venting, and with no evidence for fractionation, we do not consider that to be an issue.

The authors quote reference 25 [John, B.E., and Cheadle, M.J., 2010, Deformation and alteration associated with oceanic and continental detachment fault systems: are they similar? Geophysical Monograph v. 188, p. 175-206, 2010.] to argue for a thin ocean crust in ultra-slow spreading ridge context. But ref 25 deals with geotectonical contexts associated with detachment faults, which is not the case here. Maybe another reference would help the authors prove their case better…
Point well made -see also Rev.2. We have rewritten the relevant paragraph and updated it with more pertinent references specific to ultra-slow spreading ridges (l.310-313).